Endodontics, also known loosely as root canal therapy (RC) is a major part of dental treatment especially where saving an infected tooth is concerned. Endodontic instruments used to negotiate the canals inside the roots of teeth consist primarily of files and reamers. These instruments may be machine-driven, in which case a grasping end or shank is firmly grasped in a machine collet or chuck, such as in a rotary drill, a reciprocating filing machine or in an ultrasonic vibrating machine. Because of the sensitive nature of the root canal procedure and the small sizes of the cutting instruments, hand-held files and reamers are popular. Typically, hand-held files and reamers are provided with miniature or finger-size handles as a grasping end by which the dentist grasps the non-working end of the instrument for manipulation within the tooth nerve canal.
Files are elongated, round endodontic instruments designed for removing dentin from the inside of a tooth primarily with a vertical movement in the axial or elongated direction. Reamers are similarly elongated, rounded endodontic cutting instruments; however, they are designed primarily for enlarging or smoothing a hole in a tooth by the removal of dentin from the inside of the tooth primarily through rotation. Because of the difference in operation (i.e., an up-and-down movement, compared with rotational movement), files typically have a greater number of cutting edges and a higher twist angle and reamers have fewer cutting edges and a lower twist angle. Under the ISO standards, a file typically has four cutting edges formed as by twisting a small shaft having a square cross-section such that each of the four corner forms a cutting edge. The flat faces of the square form clearance flutes. A reamer is typically formed by twisting a triangular cross-sectional-shaped shaft such that it has three edges. The flat faces of the triangle form clearance flutes. The high twist angle of the files provides the cutting edge with a significant horizontal component relative to the up-and-down movement. The low angle of twist on a reamer provides the cutting edge with a significant vertical component for cutting by rotational movement. Many manufacturers have presented differently shaped cutting edges and flutes in order to provide different cutting characteristics.
U.S. Pat. No. 5,017,138 has, in its Background of the Invention, a summary of how endodontic instruments are standardized through the International Standard Organization (ISO). Basically, the shape, length, size and handle color of endodontic instruments are defined by ISO's Standard 3630. FIG. 1 shows a prior art endodontic cutting instruments basically according to the current ISO standards.
Pursuant to Standard 3630, the diameter of the working tip of an endodontic file (or reamer) corresponds to the number that is used to identify the file. For instance, an ISO standard size 10 means that the working tip diameter, so called "D.sub.1 " (as opposed to "D.sub.2 " which is the shank diameter further up the file), measures 0.10 mm. The next larger standard size file would be No. 15, indicating a working tip diameter of 0.15 mm at D.sub.1. The next larger standard size is 20, then 25, then 30, then 35 and so on, indicating 0.05 mm incremental size changes from one standard size to the next larger standard size until size 60 is reached. After size 60, then diameters at D.sub.1 increase by 0.1 mm for each standard size increment so that the next standard size file greater than size 60 is size 70 with a diameter D.sub.1 of 0.75 mm. Then, 80, 90, 100 and so on every 0.10 mm until size 150. However, apparently the largest size commonly available on the market is 140.
Other than the standardization of the working tip diameters and the standardization of incremental size difference between each diameter, the other noteworthy feature about the ISO standard is that it establishes an accepted international standard color scheme for the endodontic file and reamer handles. These colors are fixed and then correspond to the size of the tip diameter. Six basic colors are used, and accordingly to the ISO scheme, are repeated. The colors are: white, yellow, red, blue, green and black. Files outside of the standard range are sometimes available from various manufacturers, but there has been no accepted standard color scheme for differentiating the non-standard sizes from the standardized sizes. Some manufacturers have developed their own unique colors, and some have merely printed different size measurements on the handles. Confusion has not been avoided.
Dentists and dental assistants can still depend on the fact that an ISO size 15 file will be white in color. This is also the file that most root canals are started with. When the dentist begins to negotiate the length of the root canal completely and reach the root tip with a size 15 file or reamer without blockage or binding, the next larger file, size 20 (which is yellow in color) is grasped or handed to the dentist by an assistant for removing additional material until the tip is reached. The process is repeated, step-wise, with the next larger size 25 (which is red). Then, the size 30 (which is blue), the size 35 (which is green) and followed by size 40 (which is black). The size differences and color-coding are set forth in Table I, below.
TABLE I ______________________________________ ISO STANDARD COLOR CODE SYSTEM Diameter of Instrument Size D.sub.1 (mm) D.sub.2 (mm) Color ______________________________________ 10 0.10 0.42 Purple 15 0.15 0.47 White 20 0.20 0.52 Yellow 25 0.25 0.57 Red 30 0.30 0.62 Blue 35 0.35 0.67 Green 40 0.40 0.72 Black 45 0.45 0.77 White 50 0.50 0.82 Yellow 55 0.55 0.87 Red 60 0.60 0.92 Blue 70 0.70 1.02 Green 80 0.80 1.12 Black 90 0.90 1.22 White 100 1.00 1.32 Yellow 110 1.10 1.42 Red 120 1.20 1.52 Blue 130 1.30 1.62 Green 140 1.40 1.72 Black 150 1.50 1.82 White ______________________________________
The six colors are repeated again after size 40 (which is black) with ISO standard size 45 being white again. The standard size 50 is yellow, size 55 is red and size 60 is blue. As stated above, from standard size 15 to standard size 60, the standard incremental increase in working diameter is 0.05 mm. From size 60 to size 150, the standard incremental working diameter increase is 0.10 mm. The standard color scheme repeats sequentially for each next larger standard size. Since there is no standard size 65, size 70 is the next color in the sequence (i.e., green), size 80 is black. Size 90 initiates the repetition of the sequence at white; then size 100 is yellow, size 110 is red, size 120 is blue, size 130 is green, through size 140 which is black. Size 150 is white, but is seldom used and is seldom commercially available. From size 15 to size 140, there are eighteen different standard endodontic instrument sizes for files and reamers under the ISO system. The color scheme of white-yellow-red-blue-green-black repeats three times.
If the dentist encounters a problem negotiating the root canal with a size 15 instrument, there are now three smaller ISO-recognized standard sizes--10, 8 and 6--with corresponding working tip diameters of 0.1 mm, 0.08 mm and the very fine 0.06 mm. They bear special ISO colors of purple, silver and pink, respectively.
This color scheme is taught to dental students from the beginning of their studies in endodontics. By the time the student graduates from dental school, he or she invariably has become very familiar with the scheme for standard sizes. If a white file is encountered, he or she immediately recognizes it as either a size 15, 45 or 90. A very large ISO file would be obvious. The size difference from one standard whole size to the next (i.e., 0.05 mm or 0.10 mm increase) can be difficult to distinguish by mere visual observation without a color code. With six standard whole sizes between each repeated color, there is a sufficient size variation for most dentists and trained personnel to distinguish between each of the files of the same color even if the complete set of files becomes mixed as on a tray out of sequence.
Manufacturers of endodontic files typically color code and number their files and reamers on the handles. However, during an endodontic procedure, observation of small printed numbers is frequently obscured by materials or the dentist's fingers. It is noted that the file handles are typically only about 1 cm in length and about 0.5 cm in diameter. It is also noteworthy that the standardization of incremental increases from one whole size to the next for endodontic instruments by 0.05 mm for sizes 15 to 60 and by 0.10 mm for sizes 60 to 140 is a collective effort on the part of manufacturers and practitioners in the art. As with most well-meaning efforts, some controversy and problems have arisen. The 0.05 mm incremental increase, in some cases, has been too much. That is, sometimes where a canal can comfortably be negotiated by one standard size, it will not allow the negotiable by the next larger size which is at least 0.05 mm larger for each standard incremental size.
Recognition of this problem is not new. In the seminal book, Endodontic Practice (11th Ed.), by Grossman, et al., the authors discuss a hypothetical, yet commonly encountered, situation. In the scenario offered by the textbook, a size 10 file will go the distance of the canal but a size 15 file will not. The authors suggest that, "[b]ecause the next size file needed is not available commercially, the operator can create one by cutting off part of the instrument tip of the size 10 file. All standardized instruments taper 0.02 mm in diameter per 1.0 mm of blade length. Cutting off 1.0 mm of tip of a size 10 file converts it to a size 12 file." (p. 208) Of course, the resulting working tip, which was originally at a 75.degree. angle per ISO standards, may cut unevenly unless great skill and care is exercised to recreate a symmetrically-angled cutting tip.
More than one Endodontic textbook as well as numerous dental seminars have taught the above-mentioned method of creating "intermediate" files to the ISO system. The problem of not being able to go the distance with the next larger file has also created the problem of broken files: As the next larger file is just a little short of the length of the canal, it is not uncommon for the operator to push a little harder. The file then binds and in an attempt to pull the file out, it breaks. This has led to the oft-quoted rule in endodontics of "No more than one quarter turn" in using files and reamers.
As a result of the foregoing, many dental articles have been written about how to retrieve broken files as well as implements for sale to dentist to do same. This is also the backdrop of the U.S. Pat. No. 5,017,138.
The '138 patent criticizes the ISO's system of fixed 0.05 my incremental increase as providing a non-uniform percentage increase in size over the 0.1 mm to 1.4 mm range. The proposed solution is to offer uniform percentage increases in files rather than a fixed amount of increase. Under the proposed '138 uniform percentage increase scheme, each subsequent file would increase the same percentage in size as oppose to an irregular percentage under the ISO system.
The 0.05 mm incremental increase under the ISO mathematically does have an irregular percentage increase in file size. For instance: from size 10 to size 15, that is from 0.10 to 0.15, a 0.05 mm increase equates a 50% increase. From 15 to size 20, that is from 0.15 to 0.20, a 0.05 increase equals a 33% increase. This irregular, albeit decreasing percentage, was pointed out in the '138 patent. However, where the '138 patent fails is that even in a thirteen-instrument set (the widest selection in the examples given), for a uniform increase of 29%, after the tip size reaches 0.167 mm (instrument 5), the size increase up to the next size of 0.216 mm (instrument 6) is 0.049 mm [column 4, line 6 of the '138 patent]. We are essentially back to the ISO increase of 0.5 mm.
Above Instrument 6, the 29% increase translates to a whopping 0.063 mm between size (instruments 6 and 7): 0.216 mm and 0.297 mm [column 4, line 7]. Then a gargantuan leap of 0.087 mm. between sizes 0.297 mm and 0.366 mm (instruments 7 and 8). By the time instrument 13 is reached with a D.sub.1 of 1.293 mm. (similar to an ISO size 130 file), there is a giant 0.293 mm increase over instrument 12 [column 4, line 54]. This is three times the 0.10 mm increase under the ISO system!
Another important disadvantage of the '138 system is that it involves a totally different color scheme. At this writing, one manufacturer has offered for sale a series of 13 files based on the '138 patent. The manufacturer used its own color code in which four colors are repeated twice plus five other colors for the set of thirteen instruments. These nine new colors include several hues of brown and green. There are also two different reds, plus a pink.
It is reasonable to think that the manufacturer of the '138 set of files is trying hard to prevent any overlap of colors with the ISO colors. Imagine the dentist (or, more likely, the chairside assistant) haplessly trying to find the next larger size with file colors closely resembling one another. However, in creating new colors, the manufacturer also has unwittingly forced the user of these new files to memorize a new set of colors. Habits are hard to form and even harder to forget.
The problem with the 0.05 mm. increase under the ISO system is also drawing the attention of another manufacturer. Recently, a new set of endodontic files has been introduced which are the "half-sizes". No longer do dentists have to bring along a fingernail file in their endodontic boxes to clip file tips in order to make custom intermediate sizes. New non-standard half-sizes of 22.5, 27.5, 32.5, 42.5, and 47.5 corresponding to 0.225 mm, 0.275 mm, 0.325 mm, 0.425 mm and 0.475 mm tip diameters have been introduced. However, this only addresses part of the problem.
The problem relating to the identification of half-sizes has not been adequately addressed. These non-standard size files are in-between existing standard sizes or intermediate sizes and need to be identified properly in order that the user can quickly select the appropriate one during the root canal procedure. As with the '138 file set, a totally different color scheme will be confusing at best and will likely never be adopted or relearned. The mere printing of sizes on the small handles is inadequate in a realistic situation. More than one intermediate size in critical working ranges has not been previously proposed.